A circuit breaker is defined by National Electrical Manufacturers Association (NEMA) standards as a device designed to open and close a circuit by non-automatic means, and to open the circuit automatically on a predetermined overcurrent, without injury to itself when properly applied within its rating. A circuit breaker is also defined in American National Standards Institute (ANSI) standards as a mechanical switching device, capable of making, carrying and breaking currents under normal circuit conditions and also, making, carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of short-circuit.
There are two classifications and three types of circuit breakers used in low voltage circuit protection. The two basic classes of circuit breakers are: Low Voltage Power Circuit Breaker Class, and Molded Case Circuit Breaker Class. The three types of circuit breakers are: Low Voltage Power Circuit Breakers (LVPCB), Molded Case Circuit Breakers (MCCB), and Insulated Case (encased) Circuit Breakers (ICCB). Molded case circuit breakers are designed to provide circuit protection for low voltage distribution systems. They protect connected apparatus against overloads and/or short circuits.
The need for molded case circuit breakers was created in 1918 when numerous applications for electrical motors resulted in a demand for a device that would ensure safe operation and, at the same time, protect electrical circuits. During this period, individual motors were used for the first time in industrial plants to operate machine tools, and in private homes to operate appliances. Plant electricians were constantly changing fuses blown during motor start-ups because of the lack of properly designed fuses for motor circuit protection. Homes experienced similar problems when electrical circuits were overloaded. Inspectors were concerned about fire hazards, because of plug fuses being bridged with pennies and the installation of fuses with too high of an ampere rating. Inspection authorities became involved and attempted to find a solution to the problem. Meetings with switch manufacturers were initiated in an effort to find a solution. Switch manufacturers were asked to develop a switching device that would interrupt a circuit under prolonged overload conditions. The device would have to be safe, reliable and tamperproof. It should also be resettable so as to be reusable after an interruption without replacing any parts. This search for better circuit protection resulted in many different but unacceptable approaches to the problem. These early meetings and subsequent efforts prepared the groundwork for the eventual development of the molded case circuit breaker.